Question

How would you make 250 ml of 0.150 M sodium thiosulfate from solid Na​2​S​2​O​3​ * 5H​2​O? (molar mass = 248.2 g/mole)

Answer 1

Strength of the sodium thiosulphate solution to be produced is `0.150M=0.150"mol/"Lxx248.2g"/"mol=37.23g"/"L`

So 1L or `1000mL` solution contains `37.23g` solute.

Hence 250mL wYill contain `37.23/1000*250 =9.3075g` solute

This mass of solute is to be taken in a 250mL volumetric flask by weighing in an electronic balance and then it is disolved in water of to make a solution of volume less than 250mL and finally required volume of water is added to it to make the volume 250mL.

Answer 2

Dissolve 9.31 g of `Na_2S_2O_3 * 5H_2O` into 250mL water.

Explanation:

Dissolve 9.31 g of `Na_2S_2O_3 * 5H_2O` into 250mL water.

Molarity is moles/Liter. 0.150 M in 250mL is ¼ of 0.150 moles/L, or 0.0375 moles/0.25L.

`Na_2S_2O_3 * 5H_2O` just has the extra mass (moles) of water to consider. That is irrelevant to this problem because we are already given the molar mass for the compound. It doesn’t matter that there are a few additional water molecules hanging on, the moles of sodium thiosuflate in the compound are the same.

0.0375 moles * 248.2 g/mol = 9.3075g `Na_2S_2O_3 * 5H_2O`